The present invention relates to a device for forcing a piston ring radially outwardly and more particularly a device for forcing a piston ring radially outwardly adapted for use in a reciprocating compressor or with a piston valve reciprocating in a cylinder under the pressure of a gas or a driving valve in an air cylinder.
Generally, an oilless reciprocating compressor has a piston ring for compressing a gas. The piston ring is forced against the bore wall of a cylinder so as to minimize the leakage of the gas being compressed. Various methods have been devised and demonstrated for forcing a piston ring radially outwardly against the bore wall of a cylinder; one of them uses a plunger as shown in FIG. 5.
In FIG. 5, referece numeral 11 designates a cylinder; 12, a piston; 13, a radial hole of the piston 12; 14, a coiled spring; 15, a plunger loosely fitted in the radial hole 13 such that the coiled spring 14 is compressed; and 16, a piston ring loosely fitted into a circumferential piston ring groove of the piston 12. As shown in enlarged scale in FIG. 6, the plunger 15 comprises a rod 17 in the form of a hollow cylinder and a head 18 intergral with the radially outer end of the rod 17. A recess 19 for receiving the plunger head 18 is formed in the inner peripheral surface of the piston ring 16. More particularly, the coiled spring 14 is inserted into the plunger rod 17 and is compressed when the rod 17 is fitted into the radial hole 13 so that the plunger 15 is always pushed toward the cylinder 11. That is, the plunger head 18 is forced into the recess 19 so that the piston ring 16 is always made into intimate contact with the bore wall of the cylinder 11.
In the conventional device of the type described above with reference to FIGS. 5 and 6, the surface of contact between the plunger head 18 and the bottom of the recess 19 is flat. The head 18 and the recess 19 must be machined with a high degree of accuracy so as to avoid an offset or deviation due to misalignment.
In addition, there arises the problem that perpendicular between the surface of contact and the axis of the rod 17 must be maintained at a high degree of accuracy. That is, as shown in FIG. 7, when the axis of the plunger 15 is not aligned with the center line 20 of the recess 19, the piston ring 16 is damaged at portions indicated by A in FIG. 6 in the earlier stage of use or the joint B between the rod 17 and the head 18 is damaged in the earlier stage of use.
In addition, since the plunger head 18 is inserted into the recess 19 and is reciprocated in unison with the piston ring 16, there also arises the problem that the head 18 strikes the piston 12, leaving a dent in the piston wall.